Prior to Hoover and Coolidge boarding the C-47, the underside of the port wing is shown. Civilian registration numbers can be seen through the paint. This particular aircraft is a civilian DC-3 that has been painted over in US Air Force markings.
During the fight in the plane, two gunshots are heard before anyone draws a gun. Later, two muzzle flashes are visible (including the one that punctures the cockpit), but no sound effects accompany them.
Lieutenant Max Hoover: See, that's federal property. This isn't. This is L.A. This is my town. Out here you're a trespasser. Out here I can pick you up, burn your house, fuck your wife, and kill your dog. And the only thing that'll protect you is if I can't find you. And I already found you.